Typical materials requiring surface flatness at high level include a single-crystal silicon disk called a silicon wafer for producing semiconductor integrated circuits (IC, LSI). The surface of the silicon wafer should be flattened highly accurately in a process of producing IC, LSI etc., in order to provide reliable semiconductor connections for various coatings used in manufacturing the circuits in each steps of stacking an oxide layer or metal layer thereon. In the step of polishing finish, a polishing pad is generally stuck on a rotatable supporting disk called a platen, while a workpiece such as a semiconductor wafer is stuck on a polishing head. By movement of the two, a relative speed is generated between the platen and the polishing head while polishing slurry having abrasive grains is continuously supplied to the polishing pad, to effect polishing processing.
As polishing characteristic of a polishing pad, it is requested that a material being polished is excellent in planarity and in-plane uniformity and a polishing rate is large. A planarity and in-plane uniformity of a material being polished can be improved to some extent with a polishing layer higher in an elastic modulus. On the other hand, a polishing rate can be improved by using a foam containing pores, thereby, increasing an amount of slurry to be retained.
Considering the development of next-generation devices, there is a demand for high-hardness polishing pads capable of further increasing planarity. In order to increase planarity, hard polishing pads may also be used. If such hard polishing pads are used, however, a problem may occur in which scratches (scars) are more likely to occur on the surface of the material being polished.
Patent Document 1 proposes that for the purpose of preventing variations in life or polishing performance, a polishing plastic foam sheet should have elongated cells aligned in the in-plane direction of the sheet.
Patent Document 2 proposes that for the purpose of reducing variations in thickness and increasing polishing performance, a polishing pad should include a foamed material and have a plurality of pores in the surface part to be in contact with an object to be polished, wherein variations in thickness should be within ±15 μm, the pores should be uniformly distributed in the surface part, and the pores should have a ratio of the maximum diameter to the minimum diameter of 1.0 to 1.2.
Patent Document 3 proposes that for the purpose of increasing planarization property and in-plane uniformity, a polishing pad should include a polishing layer having closed cells, wherein the closed cells should include oval cells, and in the cross-section of the polishing layer, the oval cells should have a ratio (L/S) of average long axis length L to average short axis length S of 1.1 to 5.
Patent Document 4 discloses a laminated sheet including a base sheet and a polyurethane foam layer, wherein the polyurethane foam layer has oval cells each with a long axis parallel to the direction of the thickness of the polyurethane foam layer, and in the cross-section of the polyurethane foam layer, the oval cells have a ratio (L/S) of average long axis length L to average short axis length S of 1.5 to 3. It also discloses that the laminated sheet is a supporting sheet, a backing sheet, or a pressure-sensitive adhesive sheet.
Patent Document 5 proposes that for the purpose of increasing planarization property and in-plane uniformity and suppressing clogging and scratches, a polishing pad should include a closed void-containing polyester sheet containing polyester resin and incompatible thermoplastic resin, wherein the sheet should have a Shore D hardness of 50 or more, a compressibility ratio of 1.3 to 5.5%, and a compression recovery ratio of 50% or more, and the closed voids should have a flat shape with a long diameter of 5 to 30 μm, a short diameter of 1 to 4 μm, and a depth of 1 to 5 μm.
As mentioned above, considering the development of next-generation devices, there is a demand for polishing pads capable of further increasing planarity and making it possible to suppress scratches, but even using the above polishing pads, it has been difficult to satisfy the required planarization property and the scratch reduction at the same time.    Patent Document 1: JP-A-2003-209078    Patent Document 2: JP-A-2006-142474    Patent Document 3: JP-A-2007-245298    Patent Document 4: JP-A-2007-245575    Patent Document 5: JP-A-2009-291942